How do I get this rainbow to fade and disappear?
Me too a beginner and me also don't know much about 2d animation but I think I figured out how to do it.
After drawing the rainbow chose the frame you want the rainbow to see, then go to object data properties and make sure that opacity is at 1.000 (1). Then move your mouse pointer to the opacity and click 'I' you can see that the opacity tab is now yellow. Then chose the frame that it want to be completely disappear. Make the opacity to 0 and click 'I'. Press space bar and see it fades away.